Commonly available hydraulically actuated vehicular power steering systems lack tactile force feedback that is representative of net steering effort required by a vehicle There are two reasons for this phenomenon. First, due to various additional seals and the like, gross supplemental steering effort provided by such a system must overcome increased system friction in addition to providing a required level of net supplemental steering effort. Second, the gross supplemental steering effort is determined by deflection of a control valve which is only partially sensitive to the gross supplemental steering effort provided by the system.
The deflection of the control valve is determined by rotational deflection of a torsion bar which deflects proportionally with torque applied to a steering wheel of the vehicle This is unsatisfactory because the control valve is nominally a flow control device as opposed to a force control device. Thus a driver of the vehicle receives tactile force feedback that first relates to overcoming :he increased system friction noted above and then steering motion (velocity) rather than the net steering effort required by the vehicle.
Commonly available hydraulically actuated vehicular power steering systems are also subject to catastrophic failure should a pump drive belt break or a hydraulic line or any system component fail. While manual steering is still possible when such a system is inoperative, apparent steering effort is greatly increased. In fact, :he apparent steering effort required is significantly greater than that required by an otherwise identical vehicle equipped with manual steering. This is due to the necessity of overcoming the increased system friction noted above as well as moving hydraulic fluid against system hydraulic impedance. Such a catastrophic failure could be dangerous depending on other factors involved. For instance, a person having limited physical strength could have difficulty controlling a vehicle suffering such a catastrophic failure in a tight turn on a mountain road.
Typical hydraulically actuated vehicular power steering systems are extremely inefficient. This is because the type of pump commonly utilized in such a system is a constant displacement pump that is subject to a wide range of rotational speeds (because it is coupled directly to the vehicle's engine) while it pumps against system pressure. Even though excess hydraulic fluid is returned to a reservoir by a flow control valve, system power consumption is high whenever the vehicle's engine is operated above idle speed.
Recently, "electronically controlled" hydraulic power steering systems have become commercially available. The function of such a system is to increase apparent steering effort concomitant with increased vehicular speed. This is accomplished by utilizing an additional motor driven flow control valve which is used to siphon off a portion of hydraulic fluid flow. This serves to reduce system gain at higher vehicular speeds.
A number of electronically controlled and electrically actuated vehicular power steering systems have been described in recently issued U.S. and foreign patents. Many of these new power steering systems are said to achieve a quasi-linear operation characteristic wherein gross supplemental steering effort is related to applied steering effort. This is usually accomplished by utilizing a computer controlled electrical servo system which provides gross supplemental steering effort via an electrical servo motor drive. In the majority of these systems, the servo motor is coupled to a vehicle's steering system via mechanical means and rotates concomitantly with the vehicle's steering wheel with a linear rotational position relationship therebetween.
Most of these systems achieve control of gross supplemental steering effort via measuring torque that is applied to the vehicle's steering wheel and driving the servo motor with the computer controlled electronic servo system in a manner that is dependent upon the measured torque. This is done such that the quasi-linear operation characteristic is attained.
Many of the above mentioned patents describing electronically controlled and electrically actuated vehicular power steering systems have not included means for detecting failures of measurement devices, control electronics or actuating elements. Nor have they provided for fail-safe mechanical operation in the event of such failures. Such failure detection and fail-safe operation are minimum requirements for any commercially produced power steering systems.
Two electronically controlled and electrically actuated vehicular power steering systems that incorporate failure detection means are described in U.S. Pat. Nos. 4,660,671 and 4,715,463 entitled ELECTRIC STEERING GEAR and ELECTRIC POWER STEERING SYSTEM FOR VEHICLES, respectively. In these systems, computer controlled electronic sub-systems are used to analyze output signals from various sensors to provide failure detection. Should such analysis detect any system failure, electrical drive to the servo motor is disabled and mechanical steering is possible.
Since the servo motor in each of these systems is mechanically coupled to its respective steering system in a linearly related manner, applied steering effort required for manual operation thereof must additionally rotate all of the power steering elements. This means that the applied steering effort required for such manual operation will be greater than that required for an otherwise similar vehicle equipped with manual steering.
Two vehicular steering systems that comprise variable ratio steering are described in U.S. Pat. Nos. 4,739,855 and 4,751,976 entitled VEHICLE STEERING SYSTEM HAVING A STEERING RATIO CHANCING MECHANISM and STEERING SYSTEM FOR AUTOMOTIVE VEHICLE OR THE LIKE, respectively. In U.S Pat. No. 4,739,855, a steering ratio changing mechanism is described that increases steering ratio concomitantly with increasing vehicular speed.
The steering ratio changing mechanism incorporated therein is a planetary gear set that is utilized in a differential manner. In this planetary gear set, the vehicle's steering wheel is directly coupled to its sun gear, a stepping motor is directly coupled to its planetary carrier, and its ring gear is utilized to drive the vehicle's steering gear. The effective steering ratio varies from a relatively low ratio wherein a moderate rate of drive pulses is delivered to the stepping motor to a high ratio wherein a high rate of drive pulses is delivered to the stepping motor. Rotation of the ring gear is a function of stepping motor driven planetary carrier rotation less a function of steering wheel driven sun gear rotation. Thus, if the stepping motor or its drive circuit should fail, the ring gear will rotate in a direction opposite from the direction of rotation of the vehicle's steering wheel.
The variable steering ratio mechanism described above is said to be dependent upon a hydraulically actuated power steering system which is also disclosed. This allows the front wheels to be held at a turned position by a small force. Thus, the required holding force of the stepping motor may be relatively small. No provision is made for manual steering in the event of failure of the hydraulically actuated power steering system.
In U.S. Pat. No. 4,751,976, a variable ratio steering system is described wherein a differential planetary gear set is utilized to additively couple rotational inputs from the vehicle's steering wheel and a variable speed motor. These coupled rotational inputs drive the vehicle's steering gear.
In a first embodiment, the differential planetary gear set is an external differential planetary gear set wherein the steering wheel is directly coupled to a first sun gear of the gear set, the gear set's planetary carrier is driven by the variable speed motor, and a second sun gear of the gear set is used to drive the output shaft. In other embodiments, the differential planetary gear set is an internal differential planetary gear set wherein first and second ring gears are substituted for the first and second sun gears, respectively. In the various embodiments, gear ratios between the first sun or ring gear and the second sun or ring gear range between 0.9:1 and 1.1:1. Thus, these variable ratio steering systems substantially do not provide torque assistance.
Although it is commonly alleged that computer control of complex control functions (such as those described above) can be achieved with a relatively simple electronic sub-system, such is often not the case. This is because the computer does not work alone in providing control. Various sensor, analog-to-digital converter, digital-to-analog converter, memory and other support devices are also required. Thus, such electronic sub-systems can get very complex. In addition, software development for such computer controlled electronic sub-systems is difficult and time consuming. Furthermore, it is difficult to get such computer controlled electronic sub-systems to operate with a cycle rate that is fast enough so that overall system operation is not severely compromised.
It would be desirable to provide an electronically controlled and electrically activated vehicular power steering system wherein electronically controlled variable ratio steering, significant torque assistance and fail-safe safety systems are all incorporated. The fail-safe safety system should detect any system malfunction by simply measuring actual output parameters, comparing them with desired output parameters and disabling system drive should excessive variation occur therebetween. There should be no increase in apparent steering effort when the system drive is disabled and manual steering is undertaken. There should be a linear relationship between apparent steering effort and actual steering effort at all times. Further, It would be desirable to provide such an improved electronically controlled and electrically actuated vehicular power steering system wherein no interactive computer control is required.
Another steering system, which was disclosed in Japanese patent application First Provisional Publication No. 47-20835, was cited in U.S. Pat. No. 4,751,976. In that steering system, a steering shaft is formed in first and second sections which are interconnected by differential gearing comprising a planetary gear set. In a first embodiment of this planetary gear set, the vehicle's steering wheel is directly connected to its sun gear, a motor drives its ring gear, and its planetary carrier drives the vehicle's steering gear.
In U.S. Pat. No. 4,751,976 this steering system is said to be unsatisfactory because "it is necessary to provide a reduction gear such as a worm gear arrangement between the motor and the element of the differential gear via which the auxiliary power input is achieved . . . the provision of this gearing causes the system to become heavy and bulky thus causing design difficulties when trying to incorporate such a system into the confines of an automotive vehicle."
However, electronically controlled variable ratio steering is possible with such differential gearing. And, all members of the planetary gear set rotate in the same direction. Further, the reduction gear ratio between the sun gear and the planetary carrier results in significant steering torque assistance wherein a linear torque relationship between apparent and actual steering efforts is guaranteed. A fail-safe safety system can be effected wherein servo motor rotation is precluded and an increase in steering ratio obviates any increase in steering effort.
In accordance with the present invention, an improved electronically controlled and electrically actuated vehicular power steering system (hereinafter called an improved power steering system) is provided wherein a compact mechanical drive featuring planetary reduction gearing is utilized for coupling a servo motor to such a planetary gear set. The electro-mechanical components of the improved power steering system are mounted concentrically with respect to the steering shaft or, alternately, concentrically about a pinion gear axis within a portion of a housing of a rack and pinion gear set of the improved power steering system.
In normal operation of the improved power steering system, redundant first and second sets of shaft angle transducers measure angular positions of the steering shaft and the servo motor. In each of the first and second sets of shaft angle transducers, first and second shaft angle transducers measure the angular positions of the steering shaft and the servo motor, respectively. The values of the signals from the first (or data) set of first and second shaft angle transducers are utilized for data input to an electronic servo system. The electronic servo system drives the servo motor such that the position of the servo motor and gear train track the position of the steering shaft according to a preselected function of the value of the signal from the first shaft angle encoder. Then both the steering shaft and the gear train rotate and additively provide vehicular steering motion as desired.
The second set of first and second shaft angle transducers is utilized to provide a continuous verification of system function. This is done by comparing the value of the signal from the second shaft angle transducer of the second (or measurement) set of shaft angle transducers with the value determined according to the same preselected function of the value of the signal from the first shaft angle encoder. If these values are excessively in PG,10 variance, then a fail-safe safety system is activated.
On the other hand, if one of the steerable wheels is jammed (such as against a curb) and current applied to the servo motor becomes excessive, a current-limiting circuit concomitantly suppresses the value of the signal from the first shaft angle transducer of each set of shaft angle transducers. This serves to suppress the output of the electronic servo system and limit the current that is applied to the servo motor to a safe value. Since the value of the signal from the second shaft angle transducer of the measurement set of shaft angle transducers continues to match the value determined according to the preselected function of the suppressed value of the signal from the first shaft angle transducer of the measurement set of shaft angle transducers, the fail-safe safety system is not activated in the current limiting mode of operation.
Variable steering ratio capability is provided via the preselected function. This is accomplished by varying angular motion of the servo motor as a function of vehicular speed so that the apparent steering ratio smoothly varies from a minimum value at low vehicle speeds to a maximum value at high vehicle speeds. This is done by concomitantly suppressing the values of each of the signals from the first shaft angle transducers of each of the first and second sets of shaft angle transducers according to a pre-selected function of vehicular speed.
In a first embodiment of the improved power steering system, the servo motor is coupled to the ring gear via non-reversible (self-locking) gearing. A fail-safe safety system, which includes a contactor that electrically disconnects the servo motor and places a line fault across its armature windings, precludes servo motor rotation when activated. Thus, the ring gear is held stationary and manual steering can be accomplished with no apparent change in steering effort at an increased mechanical steering ratio.
In another embodiment of the improved power steering system, the servo motor is coupled to the ring gear via reversible gearing. A modified fail-safe safety system, which additionally includes a fail-safe servo motor brake, similarly precludes servo motor rotation when activated. Thus, the ring gear is held stationary and manual steering can be accomplished with no apparent change in steering effort at an increased mechanical steering ratio.
All electronic control functions of either embodiment of the improved power steering systems are carried out in real time by analog electronic circuits. No computer is used therewith.